Method of manufacturing fine grain ceramic barium titanate

ABSTRACT

A method of manufacturing fine grain barium titanate ceramic bodies from compositions including semiconducting barium titanate ordinarily doped with Nb2O5, Ta2O5, Sb2O5, Bi2O3, La2O3, CeO2, Gd2O3, Sm2O3, and Y2O3, and dielectric barium titanate. Barium sulfate is included as a starting ingredient in the barium titanate materials, and the materials are calcined and fired in a conventional manner. The resulting ceramic materials have uniform fine grain size.

United States atent i 1 Matsuo et al.

[451 Oct. 9, 1973 METHOD OF MANUFACTURING FINE GRAIN CERAMIC BARIUMTITANATE Inventors: Yoshihiro Matsuo, Neyagawa;

lliiromu Sasakl, Kadoma; Shlgeru Hayakawa, Hirakata, all of JapanAssignee: Matsushita Electric Industrial Co.

Ltd, Osaka, Japan Filed: Feb. 17, 1972 Appl. No.: 227,306

US. Cl 252/1, 106/46, 106/63, 106/306, 252/62.3 BT

Int. Cl. C09k 1/60 Field of Search 252/1, 62.3 BT; 106/46, 63, 306

References Cited UNITED STATES PATENTS 3,028,248 4/1962 Glaister 106/393,673,119 6/1972 Veoka et a1. 252/520 OTHER PUBLICATIONS ChemicalAbstracts 75, 52351 (Costell et al.)

Primary Examiner-George F. Lesmes Assistant ExaminerWilliam R. Dixon,Jr. Attorney-E. F. Wenderoth et a1.

[57] ABSTRACT 5 Claims, No Drawings METHOD OF MANUFACTURING FINE GRAINCERAMIC BARIUM TITANATE The invention relates to a method formanufacturing barium titanate ceramics, and more particularly to amethod of making barium titanate ceramics with smaller and more uniformgrain size.

Fine grained ceramic microstructure has long been of interest to thosein the ceramics arts. It has been well known that the grain size ofsemiconducting barium titanate ceramics has a significant effect onvoltage dependence of the resistivity anomaly (N. Hirose and H. Sasaki,J. AM. CERAM. SOC., 54[6]320(l97l A fine and uniform grain size isdesirable for a small "voltage dependence of resistivity anomaly andhigh stability during operation at high electric power loads. Thesecharacteristic properties have become increasingly important for anelectric current limitor operating at a high electric load.

The prior art discloses that the grain size of semiconducting bariumtitanate ceramics can be controlled by the addition of some oxides, forexample, SiO GeO or A1 This addition technique according to the priorart makes the grain size smaller than p, but lowers disadvantageouslythe positive temperature coefficient of electric resistance.

The method of the present invention is applicable not only tosemiconducting ceramic BaTiO but also to dielectric ceramic BaTiO whichhas a high electric resistivity and a high dielectric constant. It hasbeen known that the grain size of dielectric barium titanate ceramicshas a significant effect on temperature dependence of the dielectricconstant near the Curie temperature. A fine and uniform grain size isdesirable for small temperature dependence of a high dielectric constantnear the Curie temperature. This characteristic property has becomeimportant for a capacitor having a high capacitance, small size, andhigh reliability.

An object of this invention is to provide a method for manufacturingbarium titanate ceramics having a small and uniform grain size.

A further object of this invention is to provide a method formanufacturing semiconducting barium titanate ceramics having a largepositive temperature coefficient of electric resistivity and a smallvoltage dependence of the resistivity anomaly.

Another object of this invention is to provide a method formanufacturing dielectric barium titanate ceramics having a small grainsize which results in a small temperature dependency of the dielectricconstant near the Curie temperature.

These and other objects of this invention will be apparent uponconsideration of the following detailed description.

A method for manufacturing barium titanate ceramics according to thepresent invention comprises the steps of providing a final mixture of(a) 98.00 to 99.95 weight of a primary mixture with a compositionequivalent to BaTiO and (b) 0.05 to 2.00 weight of BaSO forming saidfinal mixture into a given form and firing the formed final mixture at atemperature of l,260C to l,400C for 0.5 to 5 hours.

Said primary mixture referred to herein can be a mixture of 50 mol oftitanium dioxide and 50 mol of barium oxide or a barium compound whichis converted into barium oxide upon firing at a high temperature, suchas barium carbonate, or can be finely divided particles of bariumtitanate, BaTiO Also included within the expression equivalent to BaTiOare compositions which are mainly BaTiO buth which have been slightlymodified by added ceramic materials or by the replacement of some of theBa or Ti atoms.

When said barium titanate ceramics are for use in a PTC thermistorhaving a positive temperature coefficient of electrical resistivity, theabove described primary mixture must be modified so that it hascomposition of 99.9 to 99.995 wt. of BaTiO and 0.005 to 0.5 wt. of oneoxide selected from the group consisting of Nb O Ta O Sb O Bi O La o CeOGd- O Sm O and Y O When it is necessary to change the Curie temperatureof barium titanat e ceramics, said primary mixture of BaTiO is modifiedin such a way that l to 35 atom of Ba or 1 to 20 atom of Ti is replacedby equivalent amount of Sr or Pb or by an equivalent amount of Sn or Zrin accordance with the prior art.

The addition of BaSO, in an amount less than 0.05 wt. to said primarymixture with a composition equivalent to BaTiO does not have anyperceptible novel effect in lowering the grain size of the bariumtitanate ceramic down to a size less than 10 microns. When BaSO in anamount higher than 2.00 weight is added to said primary mixture, even inwhich Ba or Ti has been replaced, the resultant barium titanate ceramichas an impaired dissipation factor (tano). The addition of BaSO, in anamount greater than 2.00 weight to said primary mixture which has beenmodified for use in a PTC thermistor causes the resultant bariumtitanate ceramic to lose its PTC characteristics.

The modified primary mixture is admixed with BaSO. to form said finalmixture of 98.00 to 99.95 weight of said modified mixture and 0.05 to2.00 weight of BaSO In the final mixture, the BaTiO can be composed offinally divided particles of BaTiO or of a mixture of 50 mol of titaniumdioxide and 50 mol of barium oxide or a compound which is converted tobarium oxide during firing such as barium carbonate.

Said final mixture can be prepared by two methods. One is to mixtogether all of the necessary ingredients at once by any suitable andavailable manner, such as wet ball milling. Another is to provide saidmodified mixture by any suitable and available method to calcine saidmodified mixture at a temperature of l000 to l200C and add the necessaryamount of BaSO. to the calcined mixture in order to obtain a finalmixture which is homogeneous in composition. It has been discoveredaccording to the present invention that a better result is obtained withthe lastmentioned method including the calcination process.

Each step of the novel method according to the present invention can beachieved by any suitable and available method.

The aforesaid primary mixture with the desired composition is preparedby mixing well all of the necessary ingredients by any suitable methodsuch as a well ball mill. The primary mixture is calcined in air at atemperature of l,000C to l,200C for a time period of 2 hours. Thecalcined mixture is crushed by any suitable method, such as wet ballmilling, while the required amount of BaSO, is added in order to obtainthe aforesaid final mixture having a homogeneous composition. Said finalmixture is dried, admixed with a binder such as a polyvinyl alcoholsolution, and granulated by a well known method, such as a spray dryingmethod. The

granulated mixture is pressed into a desired form such as a disc. Thepressed body is fired in air at a temperature of 1,260C to 1,400C for atime period of 0.5 to 5 hours to form the resultant barium titanateceramic.

4. E5136 seen in Table l, the grain sizes were reduced with increasingamounts of barium sulfate, and at the same time the resistivities at 20Cwere increased. The preferred amounts of barium sulfate to be added wereIf necessary, in order to make the process simple, the 5 0.05 to 2weight percent for all of the basic composicalcination process can beomitted. In such a case, said tions. final mixture is prepared by mixingtogether all the in- 14 samples (Table II) of dielectric barium titanategredients, including BaSO by any suitable and availwere prepared bystandard ceramic processing steps as able method such as wet ballmilling. described above. Samples 28, 35 and 42 were prepared byconventional processing," i.e., without the addition EXAMPLES of BaSO,,.The other samples were prepared by the pro- 27 samples (Table I) ofsemiconducting barium titacess of the invention, i.e., with the additionof barium nate were prepared by standard ceramic processing sulfateafter the calcination and in the amounts indisteps as described above.Samples 1, 10, 19 and 25 cated. were prepared by conventionalprocessing, i.e. with- The fired pellets were observed on an electronmicroout the addition of BaSO The other samples were prescope and thegrain sizes were determined. Ag elecpared by theprocess of theinvention, i.e., with the adtrodes were attached on both surfaces of thepellets and dition of barium sulfate after the calcination and in thethe dielectric constant (e) and the dielectric loss (tan amountsindicated. 8) were measured at 20C and 1 MHz. As can be seen The firedpellets were observed on an electron micro- 20 from Table II, the grainsizes were reduced with inscope and the grain sizes were determined.In-Ga eleccreasing amounts of barium sulfate and at the same trodes wereattached on both surfaces of pellets and the time, the dielectric lossat 20C was increased. The prespecific resistivity of the ceramic pelletswere measured ferred amounts of barium sulfate to be added were 0.05 ata function of temperature and applied voltage. As to 2 weight percent.

Composition of final Ceramic properties mixture (wt. percent) Finalfiring Grain Resistivity Caleination Primary condition size, (at 20 C.)Composition of primary mixture conditions mixture BaSO; m. SZ-cm.

1 100 0 30 99. 98 0. 02 20 99. 95 0. 05 10 90. 90 0. 10 8 99.977 \vt.percent of Ba'IiO and 0.023 wt. per- 1,000 C., 2 l1ours. 99. 80 0.201,350 C., 1 hour..... 8 cent of Nb305. 99. 50 0. 50 5 105 99.00 1.00 3500 98. 00 2.00 2 1000 97. 00 3. 00 l 10 100 0 25 28 99. 98 0. 02 20 4399. 0. 05 10 60 99.5 wt. percent of Ba(Tiu.o1 Sno.o3)0a and 0.5 1,1000., 2 hours.... 8&8 13300 C 11mm g 63 Wt. percent SbzOa. 99:50 6 100 99.00 1. 00 5 200 98.00 2.00 3 350 97. 00 3. 00 1 10 100 0 20 80 s d 1 1 oC l 99.35 0.05 15 99.72 wt. percent of (Ba rims TiO; an 0.28 50 210urs.... 99. O 0.10 a 10 wt. percent of LaaOa. 99. 50 0. 50 11360 1 s180 99. 00 1. 00 5 250 98. 00 2. 0O 3 700 99.6% wt. perzerfitGo Bao.aPbn.o2)T1Oa and 0.31 1,050 C., 2 l1ourS 9992 0 5 }1,300 C., 1hour...-.{ 8 27 w percen o 2 a. M 0 L 0 7 TABLE II Composition of finalCeramic properties mixture (wt. percent) Grain e Tan 6 Examplecalcination condi Primary Final firing condisize (20 C., (20 C., 0.Composition of primary mixture tions mixture BaSOr tions p.111. 1 mHz.)1 1111-12.)

28 100 0 100 1300 0. 011 99. 95 0. O5 15 1700 0. 011 99. 90 0. 10 '102000 0. 013 BaTiOr 1,100 (3., 2 hours... 99. 50 0.50 1,380 C., 1hour..." 8 2500 0.015 99. 00 1. (10 6 2900 0. 020 98. 00 2.00 4 3150 0.045 95. 00 5. 00 2 3900 0. 100 0 25 8000 0. 003 0. (1)5 10 7800 0. 0830. 0 8 7500 0.0 5 99.8 wt. percent of Ba(T1o.aaSn0. 2)O3 1,100C,., 2hours... 99 0 50 1 o 1 h 8 and 0.2 wt. percent of MnOg. 1' 00 ,360 C 98.00 2. 00 1 7000 0. 030 95. 00 5.00 0. 8 6800 0. 100 100 0 20 7400 0. 0030. 05 10 7300 0. 004 t "100 0. 005 99.8 wt. percent of (Ba0.7Sro.a)T1O31,150 0., .2 hours. 9 J O 0 a 8 L I: and 0.2 wt. percent MnOz. L400 1 g98. 00 2. 00 l. 0700 0. 030 48 95. 00 5. 00 0. 8 6500 0. 100

What is claimed is:

1. A method of manufacturing barium titanate ceramics comprisingproviding a final mixture of (a) 98.00 to 99.95 weight of a primarymixture having a composition equivalent to BaTiO and (b) 0.05 to 2.00weight of BaSO forming said final mixture into a desired shape andfiring the thus formed final mixture at a temperature of 1,260 to 1,400Cfor 0.5 to 5 hours.

2. A method as claimed in claim 1 wherein said primary mixture has acomposition, consisting essentially of 99.5 to 99.995 weight ofBaTiO and0.005 to 0.5 weight of one oxide selected from the group consisting ofNb,O Ta o Sb O Bi O La O CeO Gd O 5111 0 and Yz03. I 7

3. A method as claimed in claim 2 wherein said primary mixture iscalcined at a temperature of 1000 to 1200C and then admixed with 0.05 to2.00 weight of 82180 5. A method as claimed in claim 1 wherein saidcomposition has 1 to 20 atom of Ti replaced by an equivalent amount ofone element selected from the group consisting of Sn and Zr.

2. A method as claimed in claim 1 wherein said primary mixture has acomposition, consisting essentially of 99.5 to 99.995 weight % of BaTiO3and 0.005 to 0.5 weight % of one oxide selected from the groupconsisting of Nb2O5, Ta2O5, Sb2O3, Bi2O3, La2O3, CeO2, Gd2O3, Sm2O3 andY2O3.
 3. A method as claimed in claim 2 wherein said primary mixture iscalcined at a temperature of 1000 to 1200*C and then admixed with 0.05to 2.00 weight % of BaSO4.
 4. A method as claimed in claim 1 whereinsaid composition has 1 to 35 atom % of Ba replaced by an equivalentamount of one element selected from the group consisting of Sr and Pb.5. A method as claimed in claim 1 wherein said composition has 1 to 20atom % of Ti replaced by an equivalent amount of one element selectedfrom the group consisting of Sn and Zr.